Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

This invention belongs to the field of new energy materials, and particularly relates to a lithium iron phosphate cathode material and its preparation method. The method includes: 1) adding ferrous phosphate to a magnesium chloride aqueous solution for Mg doping to obtain a precursor; 2) preparing a composite metal salt solution, adding the precursor to the composite metal salt solution for Ti / V doping to obtain an intermediate; 3) adding the intermediate to a phosphoric acid aqueous solution and mixing it with a carbon-nitrogen organic additive and a lithium source aqueous solution to prepare a pre-product solution; 4) calcining the pre-product solution to obtain the lithium iron phosphate cathode material. This invention effectively introduces metal ions at Li and Fe sites through gradient distribution and dynamic doping of multiple ions, achieving dual-site doping, and can introduce heterogeneous anions to further optimize the material. It significantly improves the low-temperature performance and rate performance of the lithium iron phosphate cathode material, and the preparation process is highly controllable, ensuring high product quality and high material utilization.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials, and in particular relates to a lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP), due to its olivine crystal structure, exhibits high theoretical specific capacity (170 mAh / g), excellent thermal stability, long cycle life, and environmental friendliness, making it a core cathode material for power batteries (especially in the commercial vehicle and heavy-duty truck sectors) and energy storage systems. However, its intrinsic defects severely limit its performance limits.

[0003] One reason is its extremely low electronic conductivity (approximately 10^6 ohms). -9 S / cm) and lithium-ion diffusion coefficient (10 -17 ~10 -14 cm 2 The capacity of lithium iron phosphate (LFP) cathode materials is extremely limited due to their low charge / discharge rate (C / s), typically restricting applicable charge / discharge rates to below 0.3 C. At 0.5 C, the capacity drops sharply to approximately 90-92%, and at 1.0 C, the drop can reach nearly 20%, resulting in significant capacity loss. Furthermore, LFP cathode materials generally suffer from insufficient low-temperature performance, with capacity loss rates typically reaching around 50% or even higher at -20°C. This leads to poor performance of LFP cathode materials in power batteries used in cold regions. Therefore, current research and development on LFP cathode materials mainly focuses on optimizing these major performance defects. One of the most common methods is element doping.

[0004] Among them, common iron-site doping elements, such as magnesium, molybdenum, and tin / titanium composite doping, and common lithium-site doping elements, such as zirconium, nickel and other high-valence ions, as well as sodium and aluminum and other elements that improve ion diffusion ability, are mostly doped with a single element. This is because multi-element mixed doping usually results in some doping elements occupying preferential sites due to differences in ionic radii and valence state competition advantages, while some elements are repelled to the grain boundary to form inactive components. It is difficult to form effective mixed doping, and it is easy to increase the impurity phase. The overall performance is not better than that of effective doping with a single element.

[0005] In terms of doping methods, common doping methods mainly include solid-phase doping, solution coprecipitation doping, and sol-gel doping. For example, solid-phase doping tends to lead to the enrichment of dopant elements on the crystal surface, resulting in insufficient internal doping. Solution coprecipitation or sol-gel methods have extremely serious defects, especially when used for multi-element mixed doping. The pH difference of different ion precipitation will lead to very serious element segregation, and the precipitation reaction rate is very fast, measured in seconds or even milliseconds and microseconds. This means that the dopant elements cannot penetrate into the crystal lattice.

[0006] It is evident that existing doping methods have many limitations and drawbacks, and are particularly difficult to apply to the mixed doping of multiple elements. Summary of the Invention

[0007] To address the shortcomings of existing lithium iron phosphate materials in terms of rate performance and low-temperature performance, as well as the limitations in the applicability and effectiveness of existing ion doping methods, this invention provides a lithium iron phosphate cathode material and its preparation method.

[0008] The main objective of this invention is to: 1. Effectively achieve multi-element doping of lithium iron phosphate cathode materials, ensuring doping uniformity and doping effect.

[0009] II. Optimize the doping process to improve the doping effect.

[0010] Third, optimize the overall performance of lithium iron phosphate cathode materials, especially rate performance and low-temperature performance.

[0011] To achieve the above objectives, the present invention adopts the following technical solution.

[0012] A method for preparing a lithium iron phosphate cathode material, the method comprising: 1) adding ferrous phosphate to an aqueous solution of magnesium chloride, adding hydrogen chloride and an antioxidant, and continuously heating and stirring until a thin paste is formed, and then sequentially filtering, drying and pulverizing to obtain a precursor.

[0013] 2) Prepare a compound metal salt solution. Add the precursor to the compound metal salt solution, add volatile acid, and then carry out the reaction under oxygen-free conditions with continuous hot stirring. The intermediate is obtained by drying and pulverizing in sequence.

[0014] 3) The intermediate is added to an aqueous solution of phosphoric acid, and a carbon-nitrogen organic auxiliaries are added to prepare a pre-solution. The pre-solution is mixed with an aqueous solution of lithium source, and then preheated and aged to obtain an aged liquid. The aged liquid is placed in a protective atmosphere for hydrothermal treatment to obtain a pre-product solution.

[0015] 4) The pre-product solution is heated and stirred until it becomes a thick paste, then dried and calcined to obtain the lithium iron phosphate cathode material.

[0016] Preferably, the concentration of magnesium chloride in the magnesium chloride aqueous solution in step 1) is 0.05-0.10 mol / L; the atomic ratio of Fe in ferrous phosphate and Mg in the magnesium chloride aqueous solution in step 1) is 1:(0.03-0.05).

[0017] Preferably, the antioxidant in step 1) includes ascorbic acid; in the process of adding hydrogen chloride and antioxidant in step 1), the amount of antioxidant is 5-10% of the mass of ferrous phosphate, and the hydrogen chloride is added until the pH of the solution reaches 3-4.

[0018] Preferably, the heating and stirring process in step 1) is controlled at a temperature of 30-50 ℃ and a stirring speed of 20-30 rpm; the viscosity of the thin paste in step 1) is 8000-15000 cP; and the pulverizing process in step 1) is controlled to pulverize to a product mesh size ≥ 60 mesh.

[0019] Preferably, the compound metal salt solution in step 2) is a mixed solution of ammonium vanadate and titanium oxysulfate. When preparing the mixed solution, an aqueous sulfuric acid solution with a pH of 2.0 to 2.5 is used as the solvent. After preparation, the concentration of ammonium vanadate in the mixed solution is 3 to 6 g / L, and the concentration of titanium oxysulfate is 12 to 18 g / L. When the precursor in step 2) is added to the compound metal salt solution, the ratio of the amount of precursor to the amount of compound metal salt solution is 200 to 300 g: 1 L.

[0020] Preferably, the volatile acid in step 2) is hydrogen fluoride; the hydrogen fluoride is added to the solution until the pH value is 1.5 to 2.0, and then the solution is heated and stirred for 25 to 35 minutes in a low-oxygen or oxygen-free atmosphere with an oxygen content ≤0.5% VOL at a temperature of 45 to 60 ℃ and a speed of 60 to 120 rpm. The solution is then filtered, dried, and pulverized to a mesh size ≥60 to obtain the intermediate.

[0021] Preferably, the phosphoric acid concentration in the phosphoric acid aqueous solution in step 3) is 0.2-0.3 mol / L; the carbon-nitrogen organic auxiliaries in step 3) are urea and ascorbic acid in a mass ratio of 1:(0.3-0.5); the lithium source aqueous solution in step 3) is a 40-45 wt% lithium hydroxide aqueous solution; and the mass ratio of the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliaries and lithium source aqueous solution in step 3) is 1:(2.5-3.5):(0.12-0.18):(0.25-0.35).

[0022] Preferably, the temperature of the preheating and aging process in step 3) is controlled at 45–60 ℃ and the aging time is 12–18 h; the hydrothermal treatment process in step 3) is controlled at a hydrothermal temperature of 160–180 ℃ and a hydrothermal time of 8–16 h.

[0023] Preferably, in step 4), the heating and stirring process is controlled at a heating temperature of 60–80 °C until a thick paste-like product with a viscosity of 25,000–50,000 cP is obtained. Then, the product is calcined in a protective atmosphere at 600–680 °C for 6–10 h to obtain the lithium iron phosphate cathode material.

[0024] A lithium iron phosphate cathode material.

[0025] The core of the technical solution of this invention lies in the multi-level synergistic cooperation to construct gradient doping.

[0026] The most important step is to first perform a first-gradient ion doping with magnesium in step 1). Magnesium ions are a very common lithium iron phosphate doping element, which can generally reduce the migration hindrance of Li ions, thereby improving the rate performance.

[0027] However, this invention differs from conventional doping methods. In this invention, ferrous phosphate is placed under critical dissolution conditions, and magnesium doping is achieved by controlling the pH value during the continuous dissolution-crystallization cycle of ferrous phosphate. This method, unlike traditional doping methods, can achieve highly uniform magnesium doping, and the doping concentration is highly controllable.

[0028] Because during this process, as magnesium doping diffuses, Mg 2+ Partial substitution of Fe 2+ When it enters the ferrous phosphate lattice, due to Mg 2+ (0.72 Å) and Fe 2+ The different ionic radii (0.78 Å) lead to local lattice distortion and increased lattice energy. Simultaneously, the solubility is reduced due to the common ion effect, and the conversion doping of (Fe) x Mg 1-x During the transformation of Mg(PO4)2, the solubility gradually decreases as x decreases and its dissolution process is suppressed by the common ion effect. The smaller x is, the lower the solubility, which gives the local doping of Mg a clear upper limit and ensures the uniform doping of Mg in ferrous phosphate. The magnesium chloride solution concentration limited by this invention can ensure that sufficient doping elements are provided while avoiding excessively high doping ion concentrations that would cause Mg to rise rapidly on the surface and form a more insoluble shell, thus inhibiting the deep doping diffusion of Mg and preventing the problem of surface enrichment.

[0029] Based on this, the technical solution of this invention first constructs a magnesium-doped ferrous phosphate precursor with high compositional and structural uniformity. This is the key to effectively achieving subsequent multi-element mixed doping. If magnesium diffusion and precursor preparation are carried out simply using conventional solid-phase methods, solution co-precipitation methods, or sol-gel methods, Mg will first become enriched, hindering the diffusion of other elements and severely suppressing the diffusion effect.

[0030] Subsequently, the technical solution of this invention further constructs an environment for multi-element doping diffusion.

[0031] In the second step (i.e., step 2), during the doping diffusion process, magnesium preferentially occupies some Fe sites through ion exchange after undergoing the treatment in step 1), forming (Fe... x Mg 1-x The Ti 3(PO4)2 precursor, after being added to a complex metal salt solution and promoted by volatile acids, due to Ti 4+ (0.605 Å) and V 5+ (0.54 Å) Ionic radius is smaller than Mg 2+ (0.72 Å), and its bond energy with oxygen is stronger. During the thermal stirring process, it gradually squeezes out the Fe sites occupied by Mg, forcing some Mg to... 2+ The detachment of ions from their original positions, especially the presence of fluoride ions, accelerates the diffusion of tetravalent titanium ions and stabilizes the crystal lattice. Furthermore, the unique solid-liquid interface dissolution and crystallization of this invention accelerates ion exchange, and through the same common ion effect, ensures the uniformity of the composition of the intermediate products formed after ion exchange. Meanwhile, some Mg ions detach from their original positions... 2+ The ions will be in a pre-activated state. Before the subsequent conversion from the intermediate to the target component lithium iron phosphate, they are in easily migratable positions and may even partially occupy Li sites in advance. At the same time, due to the vacancy defects generated after Ti / V occupies the Fe sites, the Mg ions that are removed from their original positions after the lithium source is added in step 3) are also affected. 2+ Ions can more easily enter and fix in the Li sites of the original lithium iron phosphate to form MgLi defects, i.e., Mg 2+ In fact, the ions in the technical solution of this invention are Li / Fe dual-site doped.

[0032] Effective dual-site doping of Mg is of great significance. At the Fe site, it can stabilize the lattice and reduce the migration barrier of Li ions, thereby improving the rate performance. At the Li site, it can also expand the octahedral lattice of lithium iron phosphate, increase the Li-O bond, and reduce the diffusion barrier, thus improving the rate performance and optimizing the low-temperature performance. Simultaneously, the effective doping of tetravalent titanium and pentavalent vanadium at the Fe site can increase the conduction band electronic state density, improve the electronic conductivity of lithium iron phosphate, and reduce the migration barrier of Li ions. This comprehensive approach optimizes the electrochemical performance of lithium iron phosphate cathode materials.

[0033] Compared to traditional multi-element co-doping processes, this invention employs a gradient distribution dynamic doping method, which effectively achieves uniform distribution of multiple elements and is actually atomic-level ion exchange substitution, rather than the simple mixing of traditional methods that highly rely on element diffusion. Traditional methods require extremely high energy to provide the diffusion driving force, and the diffusion process is uncontrollable, lacks uniformity, and results in poor actual preparation effects. Furthermore, traditional multi-element co-doping processes lead to Mg... 2+ The ions are locked at the Li or Fe sites and do not form Mg. 2+ The microscopic conditions for secondary migration and diffusion doping of ions cannot achieve Mg 2+ Li / Fe double-site doping of ions.

[0034] Furthermore, although steps 1) and 2) in the technical solution of this invention are both for constructing a solid-liquid interface for dissolution-crystallization and ion exchange, the anions in the solution system can actually affect the preparation process and the properties of the product obtained.

[0035] For lithium iron phosphate, both fluoride and chloride ions are beneficial heterogeneous anions. Fluoride ions can slightly increase the lattice volume and preferentially occupy specific oxygen sites, while chloride ions can weaken the Li-O bond to a certain extent and improve crystal stability. Both can positively promote the performance of lithium iron phosphate. Fluoride ions mainly improve electronic conductivity, while chloride ions mainly enhance Li ion transport efficiency. In contrast, in the sulfuric acid system used in step 2) of the compound metal salt solution, sulfate ions can also be used as heterogeneous anions for doping. However, sulfate ions tend to form impurity phases. Although this can improve the high-temperature stability and rate performance of lithium iron phosphate to some extent, it can also lead to a rapid decline in its cycle stability. Therefore, this invention introduces hydrogen fluoride for competitive site occupation. Due to its bond energy advantage and adsorption affinity advantage, fluoride ions can preferentially occupy the anion doping sites, thereby suppressing the doping of sulfate ions. The main reason for not using hydrogen chloride is that during the hot stirring process, technicians found that hydrogen chloride is extremely easy to volatilize rapidly, resulting in a significant reduction in chloride ions. Therefore, sulfate ions are still easy to be doped into lithium iron phosphate as heterogeneous anions. Although hydrogen fluoride is also volatile, its volatilization rate is low. Because there are strong hydrogen bonds and association between molecules, its volatilization process will be effectively suppressed, ensuring that there are enough fluoride ions in the solution system to achieve anion doping. After the preparation is completed, excess hydrogen fluoride can be easily and effectively removed directly by conventional means such as drying.

[0036] The beneficial effects of this invention are: by dynamically doping multiple ions through gradient distribution, this invention effectively introduces metal ions into Li and Fe sites to achieve dual-site doping, and can introduce heterogeneous anions to further optimize the material. It has a significant effect on optimizing and improving the low-temperature performance and rate performance of lithium iron phosphate cathode materials. Moreover, the preparation process is highly controllable, the product quality temperature is high, and the material utilization rate is high. Attached Figure Description

[0037] Figure 1 This is a TEM characterization image of the product obtained in Example 1 of the present invention. Detailed Implementation

[0038] The present invention will be further described clearly and in detail below with reference to specific embodiments and the accompanying drawings. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0039] Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available or obtainable by those skilled in the art; unless otherwise specified, the methods used in the embodiments of the present invention are all methods mastered by those skilled in the art.

[0040] Unless otherwise specified, the compound metal salt solutions used in the embodiments of the present invention are all mixed solutions of ammonium vanadate and titanium oxysulfate. When preparing the mixed solution, an aqueous sulfuric acid solution with a pH of about 2.5 is used as the solvent. The concentration of ammonium vanadate in the prepared mixed solution is 5 g / L and the concentration of titanium oxysulfate is 15 g / L.

[0041] Example 1: A method for preparing a lithium iron phosphate cathode material, the method comprising: 1) adding ferrous phosphate to a magnesium chloride aqueous solution with a concentration of 0.08 mol / L according to a ratio of 2 mol ferrous phosphate to 1 L magnesium chloride aqueous solution, adding industrial hydrochloric acid to adjust the pH value to about 3.5, and simultaneously adding 10 wt% ascorbic acid by weight of ferrous phosphate, heating to 45 ℃ and continuously stirring at 25 rpm until the solution viscosity is about 12000 cP to obtain a thin paste material, filtering the thin paste material to remove the solvent and vacuum drying it at 60 ℃ to constant weight, and then pulverizing it to 60 mesh to obtain the precursor.

[0042] 2) Prepare a compound metal salt solution. Add the precursor to the compound metal salt solution at a ratio of 250 g precursor to 1 L compound metal salt solution. Pass hydrogen fluoride gas into the compound metal salt solution until the pH value reaches about 1.85. Then, heat to 50 °C under a nitrogen protective atmosphere and stir continuously at 90 rpm for 30 min. After removing the solvent by filtration and drying under vacuum at 60 °C to constant weight, pulverize to 60 mesh to obtain the intermediate.

[0043] 3) Prepare a 0.25 mol / L phosphoric acid aqueous solution and a 45 wt% lithium hydroxide aqueous solution. Mix urea and ascorbic acid at a mass ratio of 1:0.35 to prepare a carbon-nitrogen organic auxiliary agent. Take the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliary agent and lithium source aqueous solution at a mass ratio of 1:3:0.15:0.30. Add the intermediate to the phosphoric acid aqueous solution and add the carbon-nitrogen organic auxiliary agent to prepare a pre-solution. Mix the pre-solution with the lithium source aqueous solution and then preheat and age at 50 °C for 15 h to obtain an aged liquid. Place the aged liquid in a nitrogen protective atmosphere and perform hydrothermal treatment at 165 °C for 12 h to obtain a pre-product solution.

[0044] 4) The pre-product solution was heated and stirred at 75 °C until the viscosity was about 35000 cP to obtain a thick paste product. After removing the solvent by filtration and vacuum drying at 60 °C to constant weight, the product was calcined at 650 °C for 8 h to obtain the lithium iron phosphate cathode material.

[0045] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0046] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0047] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0048] The characterization results are shown in Table 1 below.

[0049] Table 1: Characterization results of Example 1:

[0050] As can be clearly seen from the characterization results in Table 1 above, the lithium iron phosphate cathode material of this invention exhibits excellent performance. Under the influence of multi-element mixed doping, both electronic conductivity and ion diffusion coefficient show a significant improvement compared to pure lithium iron phosphate. Regarding cycle stability, it can be seen that at 0.1 C rate, the capacity retention rate after 1000 cycles is as high as 96%, which is excellent. Even at 3.0 C rate, the capacity retention rate after 1000 cycles can reach over 80%, demonstrating long-term cycle stability. This is mainly due to the fact that this invention achieves gradient distribution dynamic doping of multiple elements through a specific process, giving the lithium iron phosphate cathode material dual high conductivity characteristics, while... Figure 1 The TEM and TEM-mapping characterization results of the product in this example show that the lithium iron phosphate cathode material prepared in this example achieves a micron-sized particle size and has a near-spherical regular structure. The mapping diagram shows that the compositional uniformity of the Fe-site core elements (Fe, Mg, Ti, and V) is extremely high, effectively achieving uniform multi-element mixing and doping, while effectively avoiding segregation and enrichment phenomena that easily occur during multi-element doping. Furthermore, the surface has a rich pore structure and complete secondary structure, resulting in an extremely high specific surface area, compared to approximately 12–17 μm of conventional commercial micron-sized lithium iron phosphate cathode material powder. 2The specific surface area of ​​the lithium iron phosphate cathode material prepared in this example is characterized to be as high as approximately 31.2 m² / g. 2 The figure / g indicates that the lithium iron phosphate cathode material of this invention has significant advantages in many aspects. In particular, the cycle stability and low-temperature stability shown in the performance characterization results are far superior to those of existing lithium iron phosphate cathode materials.

[0051] Example 2: A method for preparing a lithium iron phosphate cathode material, the method comprising: 1) adding ferrous phosphate to a magnesium chloride aqueous solution with a concentration of 0.08 mol / L according to a ratio of 2 mol ferrous phosphate to 1 L magnesium chloride aqueous solution, adding industrial hydrochloric acid to adjust the pH value to about 3.5, and simultaneously adding 10 wt% ascorbic acid by weight of ferrous phosphate, heating to 45 ℃, and continuously stirring at 20 rpm until the solution viscosity is about 8000 cP to obtain a thin paste material, filtering the thin paste material to remove the solvent, vacuum drying at 60 ℃ to constant weight, and then pulverizing to 60 mesh to obtain the precursor.

[0052] 2) Prepare a compound metal salt solution. Add the precursor to the compound metal salt solution at a ratio of 200 g precursor to 1 L compound metal salt solution. Purge the compound metal salt solution with hydrogen fluoride gas until the pH value reaches about 1.85. Then, heat to 50 °C under a nitrogen protective atmosphere and stir continuously at 90 rpm for 30 min. After removing the solvent by filtration and drying under vacuum at 60 °C to constant weight, pulverize to 60 mesh to obtain the intermediate.

[0053] 3) Prepare a 0.25 mol / L phosphoric acid aqueous solution and a 45 wt% lithium hydroxide aqueous solution. Mix urea and ascorbic acid at a mass ratio of 1:0.35 to prepare a carbon-nitrogen organic auxiliary agent. Take the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliary agent and lithium source aqueous solution at a mass ratio of 1:3:0.18:0.30. Add the intermediate to the phosphoric acid aqueous solution and add the carbon-nitrogen organic auxiliary agent to prepare a pre-solution. Mix the pre-solution with the lithium source aqueous solution and then preheat and age at 50 °C for 15 h to obtain an aged liquid. Place the aged liquid in a nitrogen protective atmosphere and perform hydrothermal treatment at 165 °C for 12 h to obtain a pre-product solution.

[0054] 4) The pre-product solution was heated and stirred at 75 °C until the viscosity was about 35000 cP to obtain a thick paste product. After removing the solvent by filtration and vacuum drying at 60 °C to constant weight, the product was calcined at 650 °C for 8 h to obtain the lithium iron phosphate cathode material.

[0055] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0056] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0057] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0058] The characterization results are shown in Table 2 below.

[0059] Table 2: Characterization results of Example 2:

[0060] Example 3: A method for preparing a lithium iron phosphate cathode material, the method comprising: 1) adding ferrous phosphate to a magnesium chloride aqueous solution with a concentration of 0.08 mol / L according to a ratio of 2 mol ferrous phosphate to 1 L magnesium chloride aqueous solution, adding industrial hydrochloric acid to adjust the pH value to about 3.5, and simultaneously adding 10 wt% ascorbic acid by weight of ferrous phosphate, heating to 45 ℃, and continuously stirring at 20 rpm until the solution viscosity is about 1500 cP to obtain a thin paste material, filtering the thin paste material to remove the solvent, vacuum drying at 60 ℃ to constant weight, and then pulverizing to 60 mesh to obtain the precursor.

[0061] 2) Prepare a compound metal salt solution. Add the precursor to the compound metal salt solution at a ratio of 300 g precursor to 1 L compound metal salt solution. Purge the compound metal salt solution with hydrogen fluoride gas until the pH value reaches about 1.85. Then, heat to 50 °C under a nitrogen protective atmosphere and stir continuously at 90 rpm for 30 min. After removing the solvent by filtration and drying under vacuum at 60 °C to constant weight, pulverize to 60 mesh to obtain the intermediate.

[0062] 3) Prepare a 0.25 mol / L phosphoric acid aqueous solution and a 45 wt% lithium hydroxide aqueous solution. Mix urea and ascorbic acid at a mass ratio of 1:0.35 to prepare a carbon-nitrogen organic auxiliary agent. Take the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliary agent and lithium source aqueous solution at a mass ratio of 1:3:0.12:0.30. Add the intermediate to the phosphoric acid aqueous solution and add the carbon-nitrogen organic auxiliary agent to prepare a pre-solution. Mix the pre-solution with the lithium source aqueous solution and then preheat and age at 50 °C for 15 h to obtain an aged liquid. Place the aged liquid in a nitrogen protective atmosphere and perform hydrothermal treatment at 165 °C for 12 h to obtain a pre-product solution.

[0063] 4) The pre-product solution was heated and stirred at 75 °C until the viscosity was about 35000 cP to obtain a thick paste product. After removing the solvent by filtration and vacuum drying at 60 °C to constant weight, the product was calcined at 650 °C for 8 h to obtain the lithium iron phosphate cathode material.

[0064] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0065] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0066] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0067] The characterization results are shown in Table 3 below.

[0068] Table 3: Characterization results of Example 3:

[0069] As can be seen from the characterization results in Tables 1-3 of Examples 1-3 above, the lithium iron phosphate cathode material of the present invention has extremely high performance stability, and its electronic conductivity generally reaches 9×10⁻⁶. -3 S / cm or higher, approaching 1×10 -2It exhibits excellent conductivity on the order of S / cm, and its ion diffusion coefficient is far higher than that of pure lithium iron phosphate cathode materials. Furthermore, it demonstrates outstanding cycle stability and low-temperature performance, making it highly valuable as a cathode active material for automotive power batteries in cold regions.

[0070] Comparative Example 1: Based on the preparation scheme of Example 1, this example uses the multi-element simultaneous doping method, that is, the following steps replace steps 1) and 2) in Example 1: According to the ferrous phosphate, magnesium chloride, titanium oxysulfate and ammonium vanadate in the compound metal salt solution used in Example 1, weigh the above substances in equivalent proportions. First, as in Example 1, use an aqueous sulfuric acid solution with a pH of about 2.5 as the solvent. Prepare a pre-solution with a concentration of 5 g / L for ammonium vanadate and 15 g / L for titanium oxysulfate. Add magnesium chloride to the pre-solution and stir to dissolve. Add ferrous phosphate under a nitrogen protective atmosphere (without adding antioxidants to avoid affecting the reaction of titanium oxysulfate and ammonium vanadate). Heat to 45 °C and stir continuously at 60 rpm until the solution viscosity is about 12000 cP to obtain a thin paste. Filter the thin paste to remove the solvent and vacuum dry it at 60 °C to constant weight. Then pulverize it to 60 mesh to obtain an intermediate.

[0071] After completing the preparation of the above intermediates, the intermediates obtained in this example are applied to prepare the same products as in steps 3) and 4) of Example 1, and finally the target product, lithium iron phosphate cathode material, is obtained.

[0072] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0073] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0074] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0075] The characterization results are shown in Table 4 below.

[0076] Table 4: Characterization results of Comparative Example 1:

[0077] As can be clearly seen from the characterization results in Table 4 above, the lithium iron phosphate cathode material prepared in this example still exhibits relatively superior conductivity. This is mainly due to the excellent conductivity provided by the nitrogen-doped carbon coating layer, although it decreases somewhat, it still remains relatively effective at approximately ×10⁻⁶. -3 The S / cm order of magnitude. However, regarding the ion diffusion coefficient, the product in this example shows a very significant and precipitous decrease compared to Example 1, with a decrease approaching two orders of magnitude, from 1.3 × 10⁻⁶ for the product of Example 1. -11 cm -2 / s plummeted to 6.6×10 -13 cm -2 The rate of decrease reached nearly 95%, with performance only slightly better than traditional lithium iron phosphate and similar to commercially available LMFP materials. This demonstrates that multi-element co-doping presents significant obstacles. In this example, the three doping elements Mg, Ti, and V coexist in the same doping system, making it impossible to effectively establish preferential Mg sites. Without effectively organizing the lattice and microstructure of ferrous phosphate, good doping cycling and migration induction cannot be achieved, resulting in sequential doping. For example, Ti and V have a much higher doping priority than Mg, and after doping, they quickly reach a "saturated doping" state. Consequently, the first-doped ferrous phosphate cannot further achieve Mg doping, and subsequent ferrous phosphate can only undergo Mg doping. This leads to extremely poor uniformity of dopant distribution, resulting in a complex and unevenly distributed product composed of Ti-LFP, V-LFP, Ti / V-LFP, and Mg-LFP, causing a severe decrease in ion diffusion coefficient. Simultaneously, the rate performance and cycling stability at room temperature and low temperature also significantly decrease.

[0078] It is evident that this invention is a highly effective solution for achieving multi-element doping of lithium iron phosphate. It optimizes and organizes the lattice and microstructure of ferrous phosphate by preferentially occupying Mg sites, and then further introduces Ti and V through competitive site occupation and ion exchange. At the same time, some Mg enters the easily migratable "activated state". In the subsequent lithium iron phosphate synthesis process, this part of Mg can enter the Li site to form dual-site doping, which has very significant technical advantages.

[0079] Comparative Example 2: Based on the preparation scheme of Example 1, this example uses the solid-state method for multi-element synchronous doping, that is, the following steps replace steps 1) and 2) in Example 1: according to the ferrous phosphate, magnesium chloride, titanium oxysulfate and ammonium vanadate in the compound metal salt solution used in Example 1, weigh the above substances in equivalent proportions. After the raw materials are prepared, they are sequentially processed by conventional solid-state process through wet grinding, spray drying granulation and air jet pulverization to 60 mesh to obtain the intermediate.

[0080] After completing the preparation of the above intermediates, the intermediates obtained in this example are applied to prepare the same products as in steps 3) and 4) of Example 1, and finally the target product, lithium iron phosphate cathode material, is obtained.

[0081] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0082] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0083] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0084] The characterization results are shown in Table 5 below.

[0085] Table 5: Characterization results of Comparative Example 2:

[0086] As can be clearly seen from the characterization results in Table 5 above, the preparation results in this example are similar to those in Comparative Example 1. In fact, the low-temperature performance, high-rate performance, and high-rate cycling stability are even weaker than those in Comparative Example 1. This is mainly because the traditional solid-state method cannot achieve dynamic doping with a multi-element gradient distribution similar to the technical solution of this invention. It cannot effectively control the microstructure and elemental distribution of lithium iron phosphate (including the intermediate M-ferrous phosphate). Furthermore, many dopant elements are difficult to effectively enter the lithium iron phosphate lattice, instead forming "impurities" that result in poor actual performance.

[0087] Comparative Example 3: A method for preparing a lithium iron phosphate cathode material, the method comprising: 1) adding ferrous phosphate to a magnesium chloride aqueous solution with a concentration of 0.08 mol / L according to a dosage ratio of 2 mol ferrous phosphate: 1 L magnesium chloride aqueous solution, adding industrial hydrochloric acid to adjust the pH value to about 3.5, and simultaneously adding 10 wt% ascorbic acid by weight of ferrous phosphate, heating to 45 ℃, stirring continuously at 25 rpm for 15 min, the viscosity of the solution after stirring is <200 cP, filtering the solution to remove the solvent, and vacuum drying at 60 ℃ to constant weight, and then pulverizing to 60 mesh to obtain the precursor.

[0088] 2) Prepare a compound metal salt solution. Add the precursor to the compound metal salt solution at a ratio of 250 g precursor to 1 L compound metal salt solution. Pass hydrogen fluoride gas into the compound metal salt solution until the pH value reaches about 1.85. Then, heat to 50 °C under a nitrogen protective atmosphere and stir continuously at 90 rpm for 30 min. After removing the solvent by filtration and drying under vacuum at 60 °C to constant weight, pulverize to 60 mesh to obtain the intermediate.

[0089] 3) Prepare a 0.25 mol / L phosphoric acid aqueous solution and a 45 wt% lithium hydroxide aqueous solution. Mix urea and ascorbic acid at a mass ratio of 1:0.35 to prepare a carbon-nitrogen organic auxiliary agent. Take the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliary agent and lithium source aqueous solution at a mass ratio of 1:3:0.15:0.30. Add the intermediate to the phosphoric acid aqueous solution and add the carbon-nitrogen organic auxiliary agent to prepare a pre-solution. Mix the pre-solution with the lithium source aqueous solution and then preheat and age at 50 °C for 15 h to obtain an aged liquid. Place the aged liquid in a nitrogen protective atmosphere and perform hydrothermal treatment at 165 °C for 12 h to obtain a pre-product solution.

[0090] 4) The pre-product solution was heated and stirred at 75 °C until the viscosity was about 35000 cP to obtain a thick paste product. After removing the solvent by filtration and vacuum drying at 60 °C to constant weight, the product was calcined at 650 °C for 8 h to obtain the lithium iron phosphate cathode material.

[0091] The performance of the prepared lithium iron phosphate cathode material was characterized, including electronic conductivity (25℃), ion diffusion coefficient (25℃), capacity characterization under different rate conditions at room temperature (25℃) and low temperature (-20℃), and cycle capacity retention under different rate conditions at room temperature.

[0092] Among them, the electronic conductivity was directly measured using the powder pressing four-probe method to obtain the electronic conductivity characterization results.

[0093] For other performance characterization, the lithium iron phosphate cathode material prepared in this example was mixed with Super-P and PVDF in a mass ratio of 8:1:1 and dispersed in NMP. After ball milling and uniform dispersion, it was coated on aluminum foil and vacuum dried to form a cathode sheet. Then, a lithium metal sheet was used as the anode, and a 1 mol / L LiPF6 solution was used as the electrolyte. The electrolyte solvent was EC:DMC:EMC in a volume ratio of 1:1:1. Celgard polypropylene membrane was used as the separator. After assembling into a battery, the performance was evaluated.

[0094] The characterization results are shown in Table 6 below.

[0095] Table 6: Characterization results of Comparative Example 3:

[0096] As can be clearly seen from the characterization results in Table 6 above, the ion diffusion coefficient of the product in this example also showed a significant decrease compared to Example 1. Although it was not as drastic as the decrease of over 95% observed in Comparative Examples 1 and 2, the decrease was still substantial. This is mainly because the preparation process of the precursor was changed in this example. In the preparation process of Example 1, the final solution viscosity was used to control the doping form and state of Mg. At low viscosity, the doping depth of Mg ions is small and cannot penetrate deep into the crystal lattice to achieve deep doping diffusion. In the subsequent doping process of Ti and V elements, a large amount of surface ion exchange and competitive site occupation will occur, which also prevents them from penetrating deeply. Furthermore, the doping of Ti and V is limited due to the decrease in the solubility of the components of the material after doping. Similarly, if the viscosity reaches too high during stirring, such as when the viscosity reached approximately 22,000 cP in another experiment, the preparation effect is also significantly poor. Because high viscosity indicates a higher overall material concentration, some diffused magnesium ions are prone to re-precipitate and form micelles, blocking the doping channels of Fe sites and forming surface deposits. Although the impact is slightly less than in this example preparation scheme, it still inevitably leads to a significant decrease in product performance. Therefore, for the technical solution of this invention, using viscosity as a macroscopic indicator to control Mg diffusion is a very effective and direct approach.

[0097] In summary, it can be seen that the present invention can effectively prepare high-quality lithium iron phosphate cathode materials with multi-element mixed doping using a relatively simple, low-cost and highly controllable technical solution. The prepared lithium iron phosphate cathode materials exhibit excellent performance in high-rate performance, cycle stability and low-temperature performance, especially in cycle stability and low-temperature stability, which have very significant performance advantages.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The method includes: 1) adding ferrous phosphate to an aqueous solution of magnesium chloride, adding hydrogen chloride and an antioxidant, and continuously heating and stirring until a thin paste is formed, and then filtering, drying and pulverizing to obtain a precursor; 2) preparing a compound metal salt solution, adding the precursor to the compound metal salt solution, adding volatile acid, and then continuously heating and stirring under oxygen-free conditions to react, and then drying and pulverizing to obtain an intermediate; 3) adding the intermediate to an aqueous solution of phosphoric acid, adding carbon and nitrogen organic additives to prepare a pre-solution, mixing the pre-solution with an aqueous solution of lithium source, and then preheating and aging to obtain an aging solution, and then placing the aging solution in a protective atmosphere for hydrothermal treatment to obtain a pre-product solution; 4) heating and stirring the pre-product solution until a thick paste is formed, drying and then calcining to obtain a lithium iron phosphate cathode material.

2. The method for preparing a lithium iron phosphate cathode material according to claim 1, characterized in that, The concentration of magnesium chloride in the magnesium chloride aqueous solution in step 1) is 0.05-0.10 mol / L; the atomic ratio of Fe in ferrous phosphate and Mg in the magnesium chloride aqueous solution in step 1) is 1:(0.03-0.05).

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, Step 1) The antioxidants include ascorbic acid; in the process of adding hydrogen chloride and antioxidants in step 1), the amount of antioxidant is 5-10% of the mass of ferrous phosphate, and the addition of hydrogen chloride is controlled until the pH of the solution reaches 3-4.

4. A method for preparing a lithium iron phosphate cathode material according to claim 1 or 3, characterized in that, In step 1), the heating and stirring process is controlled at a temperature of 30–50 °C and a stirring speed of 20–30 rpm; in step 1), the viscosity of the thin paste is 8000–15000 cP; in step 1), the pulverizing process is controlled to pulverize to a product mesh size ≥ 60 mesh.

5. The method for preparing a lithium iron phosphate cathode material according to claim 1, characterized in that, Step 2) The compound metal salt solution is a mixed solution of ammonium vanadate and titanium oxysulfate. When preparing the mixed solution, an aqueous sulfuric acid solution with a pH of 2.0 to 2.5 is used as the solvent. After preparation, the concentration of ammonium vanadate in the mixed solution is 3 to 6 g / L, and the concentration of titanium oxysulfate is 12 to 18 g / L. When the precursor is added to the compound metal salt solution in Step 2), the ratio of the amount of precursor to the amount of compound metal salt solution is 200 to 300 g: 1 L.

6. A method for preparing a lithium iron phosphate cathode material according to claim 1 or 5, characterized in that, Step 2) The volatile acid is hydrogen fluoride; the hydrogen fluoride is added to the solution until the pH value is 1.5-2.0, and then the solution is heated and stirred for 25-35 minutes in a low-oxygen or oxygen-free atmosphere with an oxygen content ≤0.5%VOL at a temperature of 45-60℃ and a speed of 60-120 rpm. The solution is then filtered, dried and pulverized to a mesh size ≥60 to obtain the intermediate.

7. The method for preparing a lithium iron phosphate cathode material according to claim 1, characterized in that, Step 3) The phosphoric acid concentration in the phosphoric acid aqueous solution is 0.2-0.3 mol / L; Step 3) The carbon-nitrogen organic auxiliaries are urea and ascorbic acid in a mass ratio of 1:(0.3-0.5); Step 3) The lithium source aqueous solution is a 40-45 wt% lithium hydroxide aqueous solution; Step 3) The mass ratio of the intermediate, phosphoric acid aqueous solution, carbon-nitrogen organic auxiliaries and lithium source aqueous solution is 1:(2.5-3.5):(0.12-0.18):(0.25-0.35).

8. A method for preparing a lithium iron phosphate cathode material according to claim 1 or 7, characterized in that, Step 3) The preheating and aging process is controlled at a temperature of 45-60 ℃ and an aging time of 12-18 h; Step 3) The hydrothermal treatment process is controlled at a hydrothermal temperature of 160-180 ℃ and a hydrothermal time of 8-16 h.

9. The method for preparing a lithium iron phosphate cathode material according to claim 1, characterized in that, Step 4) The heating and stirring process is controlled at a heating temperature of 60-80 ℃ until a thick paste product with a viscosity of 25000-50000 cP is obtained. Then, it is calcined in a protective atmosphere at 600-680 ℃ for 6-10 h to obtain lithium iron phosphate cathode material.

10. A lithium iron phosphate cathode material prepared by any one of claims 1 to 9.

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